1. Field of the Invention
This invention relates to ink jet printing systems, and more particularly to a device and method for detecting the failure of thermal heaters in bubble type ink jet printing systems.
2. Description of the Prior Art
Thermal drop on demand ink jet printing systems are well known in which a heater is selectively energized to form a "bubble" in an associated ink well. The rapid growth of the bubble causes an ink drop to be ejected from a nozzle associated therewith. Printing is accomplished by energizing the heater each time a drop is required at the nozzle position for a sufficient period of time to generate a gas bubble, cause the bubble's growth and cause an ink drop to be ejected from the nozzle by the action of the gas bubble. Conventionally, there are a whole array of jet nozzles closely spaced with respect to each other and the character or other form to be printed is determined by what pattern of nozzles is actuated to provide the desired configuration of printing on the printing surface.
The control of the pattern or actuation of the jets to be actuated is done by any one of a number of well known control devices which normally include microprocessors and other circuitry necessary to selectively actuate the desired pattern of heaters In this type of printing, each particular drop of ink emitted contributes to the overall configuration of the desired character which is being printed If for any reason an ink drop should not be ejected from a particular nozzle when desired, that particular portion of the character will be missing. In very high resolution printing, the absence of one or two drops may not be critical although their absence can be recognized by a trained eye. In lower resolution printing (i.e. less drops per character) the absence of a single drop becomes more critical. In any event, to insure proper functioning of an apparatus in its designed mode, it is necessary to keep most, if not all, of the individual jets operating as required to form the required character
One particular configuration of a heating element for bubble type printers includes a resistance heating element applied to a substrate with a passivation layer overlying the heating element The current is selectively applied to the elements of the various nozzles to cause the ink drop ejection. One of the causes of permanent failure of an ink jet nozzle is failure of the heater to heat the ink as desired. While there are several causes of failure of any particular heating element, far and away the largest cause is the degradation of the passivation layer thus exposing the underlying heater to the ink and the surrounding conditions which can then quickly result in a failure of the heater element. Typically the heater element just "burns out"; i.e. the heating element either fractures or breaks and goes to essentially an infinite resistance thereby preventing the passage of current. It is desirable to be able to detect the failures of individual jet nozzles as they occur; and further it is desirable to be able to determine the mode of failure. Did the nozzle fail to eject a drop of ink because the heater failed to produce the necessary bubble action? Or was there some other reason? e.g. clogged nozzles, etc. Once the failure mode has been determined necessary corrective action can be taken such as replacing the printhead in the event that a number of heater failures have occurred such that the head is deemed to provide unacceptable output (which may be only one defective heater element, or may be more than one depending on the head configuration, and the environment in which it is used) If it is not a failure of the heating element then other corrective actions such as cleaning the head may be taken. One of the advantages of including a means of detecting heater failure within an ink jet printer is that a repairable failure of the head such as a clogged nozzle can be distinguished from an irreparable failure of the printhead.
There have been several prior art proposals for detecting failure in bubble type ink jet devices which include U S. Pat. Nos. 4,550,327; 4,484,199; 4,471,298; 4,774,526; and 4,769,657. However none of these references teach or suggest a test circuit which can be interposed within the operating system of a printer for periodically checking the condition of the heaters of each of the jet nozzles individually while utilizing the system device power supply. The Hewlett-Packard printer utilizes a linear circuit to perform test functions on heaters, which is described in Hewlett-Packard Journal of October, 1988. This is not a non-linear connection and operates differently from the present device